re-export alloc_file()

Commit 3d1e4631 ("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 <[e-mail]>
Signed-off-by: Roland Dreier <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/file_table.c

···
 	ima_counts_get(file);
 	return file;
 }
+EXPORT_SYMBOL(alloc_file);
 
 void fput(struct file *file)
 {