fs/dax.c: release PMD lock even when there is no PMD support in DAX

fs/dax.c: release PMD lock even when there is no PMD support in DAX

follow_pte_pmd() can theoretically return after having acquired a PMD
lock, even when DAX was not compiled with CONFIG_FS_DAX_PMD.

Release the PMD lock unconditionally.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: f729c8c9b24f ("dax: wrprotect pmd_t in dax_mapping_entry_mkclean")
Signed-off-by: Jan H. Schönherr <[email protected]>
Reviewed-by: Ross Zwisler <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>