docs/vfs: update references to i_mutex to i_rwsem

VFS has switched to i_rwsem for ten years now (9902af79: parallel
lookups actual switch to rwsem), but the VFS documentation and comments
still has references to i_mutex.

Signed-off-by: Junxuan Liao <ljx@cs.wisc.edu>
Link: https://lore.kernel.org/72223729-5471-474a-af3c-f366691fba82@cs.wisc.edu


Signed-off-by: Christian Brauner <brauner@kernel.org>