fs: fs.h: fix a kernel-doc parameter description

Changeset 3b0311e7ca71 ("vfs: track per-sb writeback errors and report them to syncfs")
added a variant of filemap_sample_wb_err(), but it forgot to
rename the arguments at the kernel-doc markup. Fix it.

Fix those warnings:
	./include/linux/fs.h:2845: warning: Function parameter or member 'file' not described in 'file_sample_sb_err'
	./include/linux/fs.h:2845: warning: Excess function parameter 'mapping' description in 'file_sample_sb_err'

Fixes: 3b0311e7ca71 ("vfs: track per-sb writeback errors and report them to syncfs")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/7b33bbceb29ac80874622a2bc84127bb10103245.1592895969.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>