re-export alloc_file()

Commit 3d1e463 ("get rid of init_file()") removed the export of
alloc_file() -- possibly inadvertently, since that commit mainly
consisted of deleting the lines between the end of alloc_file() and
the start of the code in init_file().

There is in fact one modular use of alloc_file() in the tree, in
drivers/infiniband/core/uverbs_main.c, so re-add the export to fix:

    ERROR: "alloc_file" [drivers/infiniband/core/ib_uverbs.ko] undefined!

when CONFIG_INFINIBAND_USER_ACCESS=m.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/file_table.c

@@ -194,6 +194,7 @@ struct file *alloc_file(struct path *path, fmode_t mode,
 	ima_counts_get(file);
 	return file;
 }
+EXPORT_SYMBOL(alloc_file);
 
 void fput(struct file *file)
 {