include: name the fsmount() parameters like the man page

The fsmount() shim took its parameter names from glibc which calls the
last one ms_flags. That's misleading as it takes MOUNT_ATTR_* flags and
not MS_* flags. The kernel and fsmount(2) call it attr_flags. Use the
names from fsmount(2) and fix up the two comments in namespace.c that
called it mount_attrs and flags.

src/core/namespace.c

@@ -1809,7 +1809,7 @@ static int mount_bpffs(const MountEntry *m, PidRef *pidref, int socket_fd, int e
 
         pidref_done(pidref);
 
-        _cleanup_close_ int mnt_fd = fsmount(fs_fd, FSMOUNT_CLOEXEC, /* mount_attrs= */ 0);
+        _cleanup_close_ int mnt_fd = fsmount(fs_fd, FSMOUNT_CLOEXEC, /* attr_flags= */ 0);
         if (mnt_fd < 0)
                 return log_debug_errno(errno, "Failed to fsmount bpffs: %m");
 
@@ -3771,7 +3771,7 @@ static int unpeel_mount_and_setup_overlay(int pair_fd, const char *mount_path) {
                 return log_debug_errno(r, "Failed to receive configured overlay: %m");
 
         /* Create the mount */
-        mount_fd = fsmount(overlay_fs_fd, FSMOUNT_CLOEXEC, /* flags= */ 0);
+        mount_fd = fsmount(overlay_fs_fd, FSMOUNT_CLOEXEC, /* attr_flags= */ 0);
         if (mount_fd < 0)
                 return log_debug_errno(errno, "Failed to create overlay mount: %m");
 

src/include/override/sys/mount.h

@@ -46,7 +46,7 @@ int fsopen_shim(const char *fsname, unsigned flags);
    FLAGS with ATTR_FLAGS describing how the mount is to be performed.  */
 /* Defined since glibc-2.36.
  * Supported since kernel v5.2 (93766fbd2696c2c4453dd8e1070977e9cd4e6b6d). */
-int fsmount_shim(int fd, unsigned flags, unsigned ms_flags);
+int fsmount_shim(int fsfd, unsigned flags, unsigned attr_flags);
 #define fsmount fsmount_shim
 
 /* Add the mounted FROM_DFD referenced by FROM_PATHNAME filesystem returned

src/libc/mount.c

@@ -9,9 +9,9 @@ DEFINE_SYSCALL_SHIM(fsopen, int,
                     unsigned, flags)
 
 DEFINE_SYSCALL_SHIM(fsmount, int,
-                    int, fd,
+                    int, fsfd,
                     unsigned, flags,
-                    unsigned, ms_flags)
+                    unsigned, attr_flags)
 
 DEFINE_SYSCALL_SHIM(move_mount, int,
                     int, from_dfd,