btrfs: rename the extra_gfp parameter of btrfs_alloc_page_array()

There is only one caller utilizing the @extra_gfp parameter,
alloc_eb_folio_array().  And in that case the extra_gfp is only assigned
to __GFP_NOFAIL.

Rename the @extra_gfp parameter to @nofail to indicate that.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>