vfs: fix typo in s_op->alloc_inode() documentation

The function which calls s_op->alloc_inode() is not inode_alloc(), but
instead alloc_inode() which lives in fs/inode.c .

The typo was there from the beginning from 5ea626aa (VFS: update
documentation, 2005) - there was no standalone inode_alloc() for the
whole kernel history.

Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/vfs.txt

@@ -237,7 +237,7 @@ noted. This means that most methods can block safely. All methods are
 only called from a process context (i.e. not from an interrupt handler
 or bottom half).
 
-  alloc_inode: this method is called by inode_alloc() to allocate memory
+  alloc_inode: this method is called by alloc_inode() to allocate memory
  	for struct inode and initialize it.  If this function is not
  	defined, a simple 'struct inode' is allocated.  Normally
  	alloc_inode will be used to allocate a larger structure which